This does not seem like a classic bypassing issue. The high output voltage of the ADuM1250 is set directly by the power supply through the pullup resistors, it is not actually significantly loaded by what is happening inside of the chip. I would look in a couple of places. First, stop the I2C bus activity and look at your VDD to see if the supply is stable when the cabling is present. Next I would be very careful how you are referencing the scope probe. Avoid referencing at a bench supply, you might be picking up noise with a little ground loop made by the scope probe returns. Link the grounds across the part as a test.

This does not seem like a classic bypassing issue. The high output voltage of the ADuM1250 is set directly by the power supply through the pullup resistors, it is not actually significantly loaded by what is happening inside of the chip. I would look in a couple of places. First, stop the I2C bus activity and look at your VDD to see if the supply is stable when the cabling is present. Next I would be very careful how you are referencing the scope probe. Avoid referencing at a bench supply, you might be picking up noise with a little ground loop made by the scope probe returns. Link the grounds across the part as a test.